Patent DE-A-1,521,770 describes a process of the type indicated and comprising the injection of a polymeric material, according to the dielectric substrate, over one face of a conducting material panel, typically copper, on which some reliefs and their corresponding grooves have been made corresponding to future tracks and intermediate tracks, respectively. When the substrate has polymerised, a second face of the copper plate, opposite to the first one, is engraved with some grooves, corresponding to the grooves pre-existing on the first face, until eliminating all the copper material corresponding to said intermediate tracks. In this way, the tracks remain insulated from each other and partially embedded in the dielectric material of the substrate. However, in this process, the adhesion of the copper material with respect to the substrate is poor, so that said tracks tend to be detached. Said patent DE-A-1,521,770 is the background nearest to the state-of-the-art.
On the other hand, processes to improve the joint between an electro-conducting metallic plate, typically of copper, and a dielectric polymeric substrate or embedded in a polymer are well known in the state-of-the-art.
U.S. Pat. No. 5,861,076 describes a method to prepare multi-layer printed circuit boards involving the treatment of copper plate surfaces with black oxide, followed by an aqueous reducing solution containing sodium metabisulphite and sodium sulphite, providing greater roughness to the black oxide surface, then subjecting it to passivation. One or more plates treated in this way are then laminated with one or more polymeric substrate laminates or with polymeric contents by means of a process involving pressure and heat during a relatively long time interval. According to said increased roughness, the joining capacity between the copper plates and the substrate laminates is improved.
Nevertheless, this method involves subjecting the copper plates to a plurality of chemical processes whose purpose is to increase surface roughness thereof in order to improve adhesion. Said processes require careful control of each phase and make the product more expensive without providing an overall effective solution to the problem of weakness of the joint between the copper tracks and the substrate.